Overall, this cancer is somewhat rare. Each year, roughly 23,000 U.S. women are diagnosed with the disease,compared with 192,000 diagnosed with breast cancer, 52,000 diagnosed with colon cancer, and 79,000 diagnosed with lung cancer. However, ovarian malignancies are an especially nasty lot, killing half their victims within 5 years. Indeed, this disease accounts for more deaths than any other gynecological cancer.

The role of weight in ovarian-cancer risk emerges from an analysis of data on some 68,000 women participating in one of three major ongoing investigations:the New York (City) University Women's Health Study, the Northern Sweden Health and Disease Study in Umea, and the Diet in the Etiology of Breast Cancer Study in Milan, Italy. Only cancers diagnosed at least a year after the start of a study were included.

Of the 122 such ovarian malignancies, most showed up around 6 years into a study. Acknowledging that "this is not a big study, in terms of the overall number of cases, for ovarian cancer it's a very big study," explains Annekatrin Lukanova of the International Agency for Cancer Research in Lyon, France. She co-authored the new report in the June 1 International Journal of Cancer.

Her group investigated the cancer's risk against a woman's size, as measured in body mass index (BMI). Generally defined as an individual's weight in kilograms divided by the square of height in meters, BMI offers some accounting for height in evaluating what the bathroom scale says. (For persons who prefer to work with standard U.S. units, calculating BMI takes a few steps more. Divide one''s weight in pounds by 2.2, then divide one's height in inches by 32.37. Next, divide the first number by the square of the second.)

In general, BMIs 25 and over are designated overweight. Individuals with a BMI of 30 and over are classified as obese.

The apparent protective role of a woman's weight in the new study proved a real surprise, Lukanova told Science News Online: "We went in expecting to find an increase in risk with increasing BMI."

Putting the findings in perspective

For each woman identified with ovarian cancer, the researchers selected two other women from the studied populations who had the same age, menopausal status, and date at recruitment into the study. None of these women could have been recently taking hormone supplements, Lukanova explains, since it had been presumed that a woman's hormone status would influence her risk of cancers in reproductive organs and tissues.

The scientists then divided the women into four equal-size groups according to their BMI. By comparing women in the lowest-BMI category with those in successively higher ones, they found a steadily decreasing risk of the cancer. Those in the heaviest group, women with a BMI over 28, had only 52 percent the ovarian cancer incidence of recruits in the leanest group, those with a BMI of 23 and under.

When the researchers compared obese women (BMI over 30) with those in the leanest group, the apparent weight advantage appeared even stronger. Those in the heavy group had just one-third the ovarian-cancer incidence of their thin counterparts.

However, this trend was apparent only among participants in the New York and Milan studies. For reasons that remain unclear, Lukanova says, Swedish women appeared to derive no similar cancer protection from weight. Then again, she admits, the Swedish women were notably taller on average than those in the other groups, so their big bones contribute proportionately more to their weight and fat contributes less. If true, that might explain the cancer difference because women with more body fat tend to develop a different hormonal profile than leaner ones and ovarian cancer is a hormone-sensitive malignancy.

In the meantime, Lukanova— herself a leanie— points out that whatever benefits may trace to obesity here, this study should not lead heavy women to become complacent at how they tip the scales. Incidence rates for plenty of other cancers climb with increasing weight, she notes.

Last year, the American Cancer Society reviewed the downside of weight gain. It reported that the risk of an obese woman's developing post-menopausal breast cancer was 50 percent higher than for women of normal weight and the risk of gallbladder cancer and endometrium cancer (a uterine malignancy) was five times higher. A few studies have found signs of a link between obesity and increased risk of cancers in the kidney, pancreas, rectum, esophagus, and liver.

Excess weight is also a risk factor for heart disease, type-II diabetes, sleep apnea, and respiratory problems. Overall, the National Cancer Society observes, obesity is a primary contributor to an estimated 300,000 excess deaths in the United States each year. So there are plenty of good reasons for women with a high BMI to up their exercise regime and cut back on empty calories.